This invention relates to diagnostic medical ultrasonic imaging, and in particular to improved imaging methods and systems that selectively isolate harmonic image signals and combinations of harmonic and fundamental image signals.
Chapman U.S. Pat. No. 5,632,277 discloses an ultrasound imaging system that employs phase inversion subtraction to enhance the image. In the disclosed system, two transmit ultrasonic pulses that differ in phase by 180.degree. are focused in the same beam direction. Echo signals associated with these pulses are stored and then summed. Linear echoes destructively interfere in this summation due to the 180.degree. phase difference between the two transmitted ultrasonic pulses. At least some nonlinear echoes do not destructively interfere to the same extent, because the phases associated with nonlinear echoes do not generally differ by 180.degree.. In this way, the amplitude of nonlinear echoes (such as second harmonic echoes) can be increased relative to the amplitude of linear echoes (such as fundamental echoes) in the summed signal.